Ok, so plants are divided into orders, families and species as every other living thing. Inside the same species you got plants with slightly different characteristics, which make up a subspecies. Things tend to be even more complicated, when members of the same subspecies have slightly different characteristics and make up sub-subspecies.
Here is an example. I appologise for the latin but i do not know the english terms
Genus: Brasica
Species: Rapa
Subspecies: many
Sub-subspecies: from each subspecies you have a subsubspecies called oleifera that produces seeds high in oil.
So, the plant name is Brasica rapa ssp. oleifera
Hope this helps

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

The species is commonly regarded as the base point in taxonomy. The species is the lowest rank which it is essential to recognize for general taxonomic purposes. It is not however, the lowest rank which it is desirable and useful to recognize.

There are also general and special purpose classifications. General purpose classifications are those familiar sequences of families, genera and species. Special purpose classifications are used for special purposes, as its name implies, such as those based upon the infertility between plants; they are of interest to biosystematists, geneticists and plant breeders.

The International Code of Botanical Nomenclature recognizes five infraspecific ranks: subspecies, variety, subvariety, form and subform. Nowadays, only three of these ranks are commonly employed; subvariety and subform largely abandoned. Du Rietz defines them as follows:

Subspecies "a population of several biotypes forming a more or less distinct regional facies of a species". It is thus a geographical race, ecotype, topodeme or genoecodeme.

Variety "a population of one or several biotypes, forming more or less distinct local facies of species" It is thus a local or ecological race, an ecotype or genoecodeme of a lower order, or an ecodeme.

Form "a population of one or several biotypes occuring sporadically in a species population in one or several distinct characters" It is thus a genodeme or relatively minor genetic variant occuring mixed with other such distinct variants.

Definitions:

biotype: a genetically distinct entity

ecotype: individuals occupying a particular habitat and forming an interbreeding population which differs genotypically from other such populations; this group is capable of hybridizing with other such groups to give hybrids showing complete fertility

topodeme: a group of related individuals of a particular taxon occuring within a specific geographical area

ecodeme: a group of related individuals of a particular taxon occuring within a specific kind of habitat

genodeme: a group of related individuals of a particular taxon differing from others genotypically